How often do you guys change your bulbs? I am running 8 x 39 Watt (36") T5 bulbs with a (2) IC 660 ballasts. I usually run my lights around 5.5 hours.

I've heard people changing them every 6 months but its starting to get expensive. Usually, a bulb goes out around 6 months and is there a general rule? Also, does the total time the bulb is running matter?

Ive had mine for 10 months and I switched one of the atinics with a new one and can barley tell a difference. And mine are on for 9 hours

1 bulb usually ends up bad after 6-7 months. I end up changing all the bulbs when one goes out but I have a bunch of used bulbs as backup.

Probably the only real way to tell would be to get a PAR meter and take measurments when new and then I'd say each month there after. Then you will have to figure out what is your bottm line for PAR and change the bulbs when they hit that mark.

I used to let me MH bulbs go longer than a year. Didn't really see a diference in my corals. But I did see a difference in light when I changed them.

I change mine out every 12-14 months, but it truly depends on the fixture you are using and your PAR values over time. If one is going out every 6 months, you may have an issue with your fixture, I run 6 fixtures and almost never have bulbs expire on me.

What kind of fixture do you use?

Ice Cap SLR Retrofit kit using IC 660 ballasts. Each ballast powers (4) 39 watt bulbs. One note, is the IC ballasts overdrive the bulbs and I don't have any fans running cooling the bulbs.

You do not need to cool the bulbs, you should however be cooling the ballasts.

Interesting, the ballasts are mounted in the stand. I need to check how hot they get in their current position.

There is a reason why some fixtures work much better than others. Typically with T5 bulbs you want the actual bulb to be around 110 degrees or there abouts. That is why the hot air from the ballast is blown across the bulbs in the ATI fixture.